Summary Congenital heart defects are the most common developmental anomaly and are the leading non- infectious cause of mortality in newborns, underscoring the importance of studying the underlying causes. Our goal is to elucidate the genetic basis of these diseases. We and others have initially used Drosophila genetics to identify and define a first set of 'cardiogenic' genes that specify the heart within the embryo. The insights from Drosophila have served as a prototype for elucidating fundamentally conserved mechanisms of cardiac development throughout the animal kingdom. More recently our lab has begun to study how the heart tube is assembled and how heart function is established after the initial heart specification events. In this pursuit, we have discovered that the cardiogenic network of transcription factors, encoded by homeobox-, GATA- and T-box genes (eg. tinman/Nkx2-5), in addition to orchestrating the initiation of heart development, is again playing a critical role in establishing and maintaining healthy heart. Importantly, their human homologs also take part in the regulation of cardiac morphogenesis and functional properties. It is thus critical to understand the genetic relationships and functional requirements of these genes (and their interactors and effectors), not only during development but also in the fully formed mature heart. The proposed studies are expected to provide fundamental insights into human heart disease mechanisms and uncover new heart disease candidates, with particular focus on polygenic modulators of the core cardiogeneic network of transcription factors. As polygenic heart disease mechanisms are very difficult to study in mammalian systems, the proposed approaches in the Drosophila heart during morphogenesis and in the adult heart are highly significant and promising. The outcome is likely to serve as an important paradigm for identifying new heart disease mechanisms. In Aim 1, we plan to determine the network of interactions between the core set of cardiogenic transcription factors in regulating various aspects of heart function and determine the rescue capacity of potential effector genes (Aim 1a). We also plan to study the role and genetic relationships of candidate heart genes, based on their interaction with tinman and pannier (Aim 1b&c). In Aim 2, we plan to study the genetic mechanisms of cardiac morphogenesis and heart tube assembly, based on our previous findings with Nmr/Tbx20 and tinman interacting genes (such as Cdc42). First, we will examine the role of the tinman interactor Cdc42. Based on the emerging concept of parallel functions in cardiogenesis and in maintaining heart function, we will examine other screen candidates (from aim 1) for their role in cardiac morphogenesis, similar to that of Cdc42.